Elimination reactions of ions in the gas phase–I: Elimination of water and acetic acid in epimeric borneols and bornylacetates

Loss of water and acetic acid in the epimeric bornyl alcohols and acetates are shown to be analogous processes by specific ²H-labelling. Among several competing mechanisms cis-1,2-elimination is the most important. Mctastable ion characteristics of this specific process exhibit some unusual features and its product is shown to have a different structure from that produced by the competing reactions.     

Enantioselective synthesis of (+)-α-allokainic acid by asymmetric lewis acid-mediated intramolecular ene reaction

-(+)-α-Allokainic acid ((+)-1) has been prepared for the ester of cis-β-chloroacrylic acid and (-)-8-phenylmenthol by a sequence of four synthetic operations. The crucial step 8d→14d (Scheme 4) is a 1?00%, diastereo- and 90% enantioselective intramolecular ene reaction proceeding at 35° on treatment of 8d with Me₂A1C1. Saponification of 14d regenerated the auxiliary chiral alcohol and yielded (+)-1 on subsequent decarboxylation. In the analogous cyclization 11d→15d the sense of asymmetric induction (78% d.e.) was opposite as confirmed by the conversion of 15d to (-)-α-allokainic acid.     

Eliminierungsreaktionen an Kationen in Gasphase II Essigsäureeliminierung aus Androstanylacetaten

Elimination of acetic acid from the molecular ions of the epimeric androstanylacetates is investigated applying specific ²H-labelling and analysis of metastable transitions. Only 17β-androstanylacetate eliminates acetic acid in a 1,2-fashion to an extent of 16%. This contribution is shown to be a specific cis-elimination and after comparison with water-elimination in the corresponding alcohol a McLafferty type mechanism is attributed, which then appears to require a planar transition state.     

Highly Stereoselective Total Syntheses of (±)-Chelidonine and of (±)-Norchelidonine by an Intramolecular o-Quinodimethane/Nitrostyrene-Cycloaddition

Conversion of 2-bromomethylstyrene 22 and benzocyclobutenyl carbamate 28 to the benzophenanthridine alkaloids (±)-chelidonine ( 1 , five steps, 25% from 28 ) and to (±)-norchelidonine ( 2 , six steps, 24% from 28 ) are described. The key step 29 → 31 involves a highly regio- and stereocontrolled intramolecular Diels-Alder reaction of the (E)-quinodimethane 30 .     

On the Number of Rational Points of Bounded Height on Smooth Bilinear Hypersurfaces in Biprojective Space

Asymptotic formulae for the number of rational points of boundedheight on flag varieties have earlier been established. In thepaper these asymptotic formulae are recovered by a new methodfor varieties in biprojective space defined over Q that areisomorphic to the flag variety of lines in hyperplanes. The result is obtained by an application of Heath-Brown's newform of the circle method. It serves as a pointer to the investigationof rational points of bounded height on varieties in multiprojectivespace.     

Rational points of bounded height on Del Pezzo surfaces of degree six

LetK be a number field. Denote byV ₃ a split Del Pezzo surface of degree six overK and by ω its canonical divisor. Denote byW ₃ the open complement of the exceptional lines inV ₃. LetN _{W s}(−ω, X) be the number ofK-rational points onW ₃ whose anticanonical heightH _−ω is bounded byX. Manin has conjectured that asymptoticallyN _{W 3}(−ω, X) tends tocX(logX)³, wherec is a constant depending only on the number field and on the normalization of the height. Our goal is to prove the following theorem: For each number fieldK there exists a constantc _K such thatN _{W 3}(−ω, X)≤c_KX(logX)^3+2r , wherer is the rank of the group of units ofO _K. The constantc _K is far from being optimal. However, ifK is a purely imaginary quadratic field, this proves an upper bound with a correct power of logX. The proof of Manin's conjecture for arbitrary number fields and a precise treatment of the constants would require a more sophisticated setting, like the one used by [Peyre] to prove Manin's conjecture and to compute the correct asymptotic constant (in some normalization) in the caseK=ℚ. Up to now the best result for arbitraryK goes back, as far as we know, to [Manin-Tschinkel], who gives an upper boundN _{W 3}(−ω,X)≤cX^l+ε. The author would like to express his gratitude to Daniel Coray and Per Salberger for their generous and indispensable support.     

High Acceleration and Improved Diastereoselectivity of Intramolecular Ene-Type Reactions by Diethylaluminum Chloride. Preliminary Communication

The pyrrolidines 2 and 10 were obtained by thermal ene-reactions at +70° and +180° from the (Z)-4-aza-1, 6-diene 1 and from the (E)-4-aza-1, 6-diene 9 in the ratios of 75:25 and 50:50, respectively. On the other hand, these cyclizations proceeded readily in the presence of diethylaluminum chloride at ? 78° and ? 35° giving in high yield the trans-pyrrolidine 2 from 1 with 100% and from 9 with 89% diastereoselectivity.     

Elimination reactions of ions in the gas phase. V—elimination of water in hydroxybornanes and related compounds

Results of an investigation of deuterium labelled methylated norborneols and dihydrocarveols are presented to show that the 1,2 elimination of water in borneol and isoborneol is triggered by an initial ring cleavage reaction. Other eliminations of water in borneol and isoborneol are elucidated by specific and stereospecific deuterium labelling in different positions.